Flame-proofing of textiles



Feb. 12, 1957 A. BERGER 2,781,281

FLAME-PROOFING OF TEXTILES Filed May 23, 1955 Textile MaterialFlame-Pmofed with the aid of (I) Reaction product of urea and 0-containing compound of phosphorus, and

(2) Methylolmelamine INVENTOR ALFRED BERGER BY WM,

ATTORNEYS p egnj n be hs e 1 95 1 6 "foret littlelsuitedespecially'forthe treatmentof la e an undesirable way;

textile fabrics can be rendered flame-proof; wliilef'a ing the abovedrawbacks to a great'extent by "t lj t abq 11 48'%j y., ve b a fro bout24m 38 b we if m 'st tri'a zine, the ratio of component (1),. toco'fn"labbut vtwo hours. fln case the mixtureis "temperature in the n "g!period is advantageously} or even 8: v

i p v dii e'bs fi 's v iw i stint 2.1 l concentrated.stockisolutions.

1 pounds. of 224.:6-triamino-l :3 z-triazine finallymethylolcompounds.which,are partiall etherit ied 227 1 :ELAME-PRQOFWG,0F TEXTILES A r -B L tae l; Sw tze land, s gn r o b ited, Basel,Switzerland, a Swiss firm Application May 23, (1955, Serial No. 510,53331 cla m (C nk-135) This is a continuation in part of my U. S. patentapplication Serial No. 301,377,;filed July 28, 1952, now am de Know pros e fo /r nde n ex e-pt o t h t jdi a van gth ej i 11y: sable an batchesof texti leflfabrics, or thetenslile' the feel of the textiles arechanged th ,The present invention is based on the observati on that ingthe fabric with an aqueous l iquidwhich, on ins lp 'e r'ably gobtainedby heating an jexcess "of u'rea ether an .oxygen-co'ntaining" compoundoffphos rus s ected from the'group consisting bf ort'lio-p liosphor'id,acid, jpyr;o-

phosphoric acid, meta-phosphoric acid and ammonium salts of these acids,at a high temperatureg'and (2) fro n about 2 to 18% by weight, andpreferably-from 'bo' t'4 to 12%.,by weight of'a'methylol compound of aranging from 3:1 to 6zl, and drying the fabric at a raised temperature.

The reaction mixture used as component (1) may be prepared by heating anoxygen-containing compound of phosphorus of the group consisting ofortho -phospl oric acid, pyrophosphoric acid, metaphosphoric acidandammoniuni salts of said,acids, together anexces pf Qurea preferablyin a. weightra'tio, of 3 parts oftthe ,to S 'to Sfpafts ofithellatter'at 'a,temperature,p abput 100"? 'c. analzooz c.,1pI fr B1y at 155" 160130.

H 1 i P i s m l a gissfr a qsts me p ss t fi 'b re lus j As;inetliylolfi compounds 7 .of .an aminotriazine used; as

component "(2) there maybe used the methylol-,cgm-= ine); and

@156; me'thylol compounds of thosel de A s tof m elamine'vwhichtstiil'contain at least one am group and l adyantageously at least 1 two,amino, groups; J for fexample,

methylolwcompounds .of s.mel am,,,rnelem, ammeline; am-

mlide of "m ss b it t dam s t iazin a has .2- chloro-4z6-diamino-Iz3z5triazine;" and also methylol compounds of guanaminessuch, for example,as those of benzo-guanamine- (2-pheny l -4:6- diarnino l: 3 :5.- tria zine),

,aceto' guanamine Kl-methyl 4:6 amino. t or fo rrno-Igu anamine' (4,:6-diamino- 1'; 3 :5 -t ne) with aliphatic alcoholsflof low, molecularweight such as groups, inwhich 2 to 6 'inethylol groups'are et herified.

:: :1 Patented t 2 ad ant e y methy p zmelamin a e u d 4 1M52 cially sewh c a .o 'un imit olu lit a Very suitable condensationproducts arethose of unlimited solubility in Water which are obtainable from onemolecular proportion of melamine and about 2"to 3 molecular "proportionsof "formaldehyde. ,Obviously .there may-alsoibe .usedmixturesof'suchaminotriazine c'ondensationproducts'. W

lFor "preparing the aqueous impregnating liquids the individual;components may be dissolved or dispersed ..in .'water, whereby-clearsolutions or very finely ,divided .disp'ersions are obtained.

tThe 'textileifabricsrriay be impregnated with therbath by means of theknown methods and'apparatusIforimpregnatingf fabrics. .There areadvantageously used cbn- "centrated impregnating baths which arerendered slightly alkaline,';for example, by adjustment to a pH value of8. In this manner the impregnation produces1a -large increaseiih theweight of the fabric, so thatgthefiamej'proof effectds considerablyenhanced. .Wetting' agents may alsotbe'added to tlieibath'in' order toimprove :the wetting o'f the fabricHQThere are advantageously usedbathsthaviiigatdry content of about 30-50 'percent.v-Siichtbathstare'stable 'for long periods, for example, from 4. hoursto several" days, that is to say, no'wat'er- 'insohiblefcompuurids'areformedwhen thef'baths areallowed' tol'stand "at ordinary/temperature",orqif insoluble compounds were formed at the outset the amount thereofjincreases slowly and .theiundissolved portions remain ina"finely..dispersed.form. ,Duringthe drying'of the'rfab'ric at "a raisedtemperature .there are :f'oin1ed-:from-the various components of"the'iinpregnatiiigi bath" andprobably also with the assistance of thetextile fibers, insoluble compoundswhich produce .theifiame-proof cited.The StrengthWitlfWhich th'efin'ishis fixed onithe fibrousmaterialideperids .onith'e td'r'ying temperature? If tlie' tlry- 'ing'iscarried out at a temperature below C.'; for example, a'til70" VCQ, the.flame-proof: finish is 'resistant'to water and thefeel and tensilestrength of,'for example; a cotton fabricgiare notxz'hanged; "I'f'thedrying iscarried out atja temperature" above 100 C.,"for example; at-160 C., or if 'thlfabric' is .dried at a' temperature beloWlOO. C.,aindlthen after hardening is broughtabout at, for. example, 12()'-"160.Cl'; theifi'ame proo'f finish" is ..also resistanttow'ashing withneutral washing" agents. (,One or other method of dfying'is useddependingon the L purpose for whili' the treated fabric is t'obe'ii'sed'. The quality of the fiame-proofifinish depends not'onlyo'n-the conditions of drying or hardening but, as stated above, also onthe amount of theifnpregnating substances fixed on :th rfibcr.,jExpejrirnents have shown fth-at it is oPadvantage to carry out theimpregnation in such manner nfil a n rease i iwei ht of not,substantially less than about 6 percent; occurs. At increases in weightranging from .-l 0715 percent. there "are. obtained very good flametprooieffectsT. 'Dh'e impregnation 'may also bec'arriedout moreZsjtrongly so as'jto produce, for examplefanfinc'rease in weight ofabout 20 "percent; '(The iii'crease's'in" :ei'ght arecalculated'on'thematerial after it has been "dried, hardened, rinsed andfagain"dried; the untreated :fa b ric "serving as the s'tar'ti ngfpbint) 'fTheuantity :of the phosphoi' usfixed "on and iii the'fibjers i s' eachfiime" bout aasmsr illustrates textile material flame-proof'ed according tothe Example 1 15 parts of pyrophosphoric acid and 80 parts of urea areslowly heated to 140 C., and at 140-145 C. a further 15 parts ofpyrophosphoric acid are added, the temperature is raised to 160 C. andthis temperature is maintained for 30 minutes. The mixture is thenallowed to cool to about 100 C. and 90 parts of water are added to thereaction mixture, whereby a clear solution is obtained. The whole isthen allowed to cool to -15 C., whereupon a small amount of biuretcrystallizes out. The mixture is filtered to remove this precipitate,and the filtrate is mixed with ammonia until the solution has a pH valueof 8. The solution is stable at ordinary temperature and can be used forthe preparation of impregnating baths.

An impregnating bath (containing about 36% of the above reaction mixtureand 9% of the component (2) described below) is prepared by mixing 80parts of the above described concentrated solution, with 10 parts of acondensation product of unlimited solubility in water from 1 molecularproportion of melamine and about 2 to 3 molecular proportions offormaldehyde (component (2)), and 20 parts of water, a small amount of aprecipitate being formed which remains in a fine state of dispersion.

Cotton is impregnated at ordinary temperature with the above bath, andthe cotton is squeezed, then dried at 70 C. and subjected to aheat-treatment at 150 C. for five minutes. It is then thoroughly rinsedwith cold water and again dried.

Analysis shows a phosphorus content of 2.85% and a nitrogen content of5.25% by weight based on the dry weight of the untreated textile fabric.The flame-proof effect is very good. The material does not after flameor after glow. The tensile strength is not affected by the treatment andthe feel of the material remains unaltered.

Instead of the above mentioned heat treatment at 150 C., the fabricdried at 70 C. can be impregnated a second time and dried again at 70 C.

The treated material of this and the subsequent examples is tested forits flame-proof quality by means of the vertical Bunsen burner test.This test is described for example, in the book by R. W. Little(Flame-Proofing of Textile Fabrics, Reinhold Publishing Corporation, NewYork, 1947, page 111 et seq.).

Example 2 A cotton fabric is impregnated in a bath having the followingcomposition: 20 parts of water, 10 parts of a condensation product ofunlimited solubility in water from 1 molecular proportion of melamineand about 2 to 3 molecular proportions of formaldehyde, (component (2)80 parts of a solution obtained by dissolving about 30 parts of thereaction mixture (component (1)) described below in 50 parts of water.

The impregnating bath thus contains about 27% of component (1) and 9% ofcomponent (2).

After squeezing the fabric, it is dried at 70 C., subjected to aheat-treatment at 160 C. for 3 minutes and then rinsed with water andagain dried. Analysisshows a phosphorus content of 2.53% and a nitrogencontent of 4.85%.

The flame-proof finish so obtained is resistant to a washing treatmentcarried out at 70 C. for 30 minutes in a bath containing 1 gram perliter of a sulfonated washing agent having a neutral reaction. Washingcan also be carried out at the boil when a washing agent free from ionsis used.

The above mentioned reaction mixture (component (1) is made by heating30 parts of pyrophosphoric acid with 50 parts of urea for 30 minutes at160 C.

Instead of the above mentioned condensation product of unlimitedsolubility in water prepared from 1 molecular proportion of melamine and2 to 3 molecular proportions of formaldehyde, there may be used such acondensation product which is soluble in water only in the ratio of 1:1,or there may be used a condensation product of unlimited solubility inwater prepared from 1 molecular proportion of melamine and 4 molecularproportions of formaldehyde.

Example 3 The procedure is the same as that described in Example 2,except that the fabric after being dried at C. is subjected to a heattreatment at 120 C. for 20 minutes. Analysis shows a phosphorus contentof 1.30% and a nitrogen content of 3.31%.

Example 4 200 parts of urea are melted at 140 C. parts of diammoniumortho-phosphate are gradually introduced while stirring. The mixture isthen heated to 160 C. and maintained at that temperature for 30 minutes.There is obtained a clear melt. After cooling to C., 200 parts of waterare added. When the solution is cooled to room temperature a smallamount of insoluble matter precipitates and is removed by filtration.The filtrate is then adjusted to a pH value of 8 by means of ammonia.

An impregnating bath is prepared by mixing 80 parts of the abovedescribed concentrated solution with 10 parts of a condensation productof unlimited solubility in water from 1 molecular proportion of melamineand about 2 to 3 molecular proportions of formaldehyde, and 20 parts ofwater.

This impregnating bath contains 38% of the reaction mixture and 9% ofthe said methylol melamine.

Cotton is impregnated with the above bath at ordinary temperature. Thenit is sequeezed, dried at 70 C., subjected to a heat-treatment at C. for10 minutes, rinsed with water and dried. Analysis shows a phosphoruscontent of 1.77% and a nitrogen content of 4.13%.

Example 5 250 parts of urea and 100 parts of orthophosphoric acid of 75percent strength are heated at C. A further 100 parts ofortho-phosphoric acid of 75 percent strength and 50 parts of urea areadded to the reaction mixture at that temperature, and the mixture isthen heated for 10 minutes at 160 C. After cooling to 110 C., 400 partsof water are added and the solution, after being cooled to roomtemperature, is adjusted to a pH value of 8 by means of ammonia.

An impregnating bath is prepared by mixing 80 parts of the abovedescribed concentrated solution with 10 parts of a condensation productof unlimited solubility in water from 1 molecular proportion of melamineand about 2 to 3 molecular proportions of formaldehyde, and 20 parts ofwater.

This impregnating liquid contains 32% of the above reaction mixture and9% of the said methylol melamine.

The procedure is the same as that described in Example 4, except thatthe heat-treament is carried out at 140 C. for 6 minutes. Analysis showsa phosphorus content of 2.09% and a nitrogen content of 4.9%.

the temperature increasing to 160 C. When the addition Example 6 400parts of urea are melted and at 140' C. parts of meta-phosphoric acidare slowly introduced into the melt,

Example 7 Inorder to improve its absorptive capacity wool gab- -ardineis treated for 30 minutes -at '45-.50 C. in a bath which contains-1cccr-of ammonia solution of ZO-percent =st-rength; and 'l -gram-'- of asulfonated washing agent per liter, and the fabric is rinsed and dried.The fabric is then impregnated with the bath described in Example 1,dried -at"80 C., again impregnated and dried for 30 minutes at '80" C.,and then Wellrinse'd.

"The wool so treated has a very good flameproof property. Thefeelof the"fabric is not changed by the treatment.

What is claimedis:

1. A process-for renderingtextile fabrics'flarneproof, which comprisesimpregnating the fabric in an aqueous liquid which contains (1)about'12-48%by*weight of a reaction mixture obtained by heatingan-excess of urea together with an oxygen-containing compoundofphosphorus selected from the group consisting of orthophosphoric 1acid, pyrophosphoric acid, meta-phosphoric acid and armnoniumcsalts 'offtheseflacids, ,at a high temperature, and (2) about 2-18% bytweight ofajmethylol- .tm'elarninefthe ratio of component (1): to component (2)ranging from 3:1 to 6:1, and .dryingthe impregnated 1 fabric, at araised temperature.

-2. "A process for rendering textilefabrics fiameproof, -which'comprises impregnating the. fabric in an aqueous liquid whichcontains 1) about 24-38% by weight of a reaction mixture obtainedby'heatin-g an excess of urea I together: with an oxygen-containing;compound ofephosphorus selected from the group consisting oforthophosphoric acid, pyrophosphoric acid, meta-phosphoric acid andammonium salts of these acids, at a high temperature, and (2) about4-12% by Weight of a methylolmelamine, the ratio of component (1) tocomponent (2) ranging from 3:1 to 6:1, and drying the impregnated fabricat a raised temperature.

3. A process for rendering textile fabrics fiameproof, which comprisesimpregnating the fabric in an aqueous liquid which contains 1) about24-38% by weight of a reaction mixture obtained by heating an excess ofurea together with pyrophosphoric acid at a high temperature, and (2)about 4-12% by weight of a methylol-melamine the ratio of component (1)to component (2) ranging from 3:1 to 6:1, and drying the impregnatedfabric at a temperature ranging from 70 C. to 160 C.

4. A process for rendering textile fabrics flameproof, which comprisesimpregnating the fabric in an aqueous liquid which contains (1) about27-37% by weight of a reaction mixture obtained by heating -8 parts ofurea with 3 parts of pyrophosphoric acid at 160 C. for 30 minutes, and(2) 9% by weight of a condensation product of unlimited water-solubilityof one mol of melamine with about 2-3 mols of formaldehyde, drying theimpregnated fabric at 70 C. and after-hardening the dried fabric atISO-160 C.

5. A process for rendering textile fabrics flameproof, Which comprisesimpregnating the fabric in an aqueous liquid which contains (1) about24-38% by weight of a reaction mixture obtained by heating an excess ofurea o ether with d ammea um .at hg hosp a e a a hi vternperature,.and(2 aboutA-l-12% by weightof ameth- -y1ol-melamine, the ratio ofcomponent (1) to component r gi o Lan -dryin h impr gnats fabricatatemperature ranging from 70 C. to. 1I60 C. .6. A process ;for renderingltextile fabrics flameproof, which comprises impregnating the fabric inan aqueous liquid which contains 1) 3.8%, by..weight,of,areactionmixture obtained by,he ating,;6.partsofiureawith 3 parts ofdiammoniumortho-phosphateat 160. C. for.3 01rninutes, and (2)9%Ibylweight of a c ondensationprodnct of unlimited water-solubility,ofone, molof, melamine. with about 2-3 molsof formaldehyde, drying-theimpregnated fabric, at 70 c. andafter-h ar-dening t dried fabric at 7. Aprocess for renderingtextile;fabrics flarneproof, which-comprisesimpregnating the fabric in an, aqueous liquid-which, contains,(1),.abput 241- 3876 byweight of .a reaction mixture obtained by heatingan excessof urea togetherwith ortho phosphoric acidsat a-hightemperature, and (2) .fabout-.,4- '12%. lay-weight of 1amethylolmelamine, the ratioof component: 1) -to compone nt (2) rangingfrom 3:1,to 6:1, andfirying,theimpregnated fabriofit a temperatureranging; from .70 C. to 160. C.

"8. A process for re ndering textile fabrics-fiameproof, which comprisesimpregnatiiigdhe fabric in an, aqueous liquidwhich contains '(1) 32%..,by:Weight of a reaction ,mixture obtained by heatingd parts ,of.ureawith B parts of ortho-phosphoric acid at 1,60"v C. for 10 minutes,and (2) 9% by weight ,of. a, condensation product .of unlimitedwater-solubility of one mol of melamine-with "about 2-3 ,rnols of,formaldehyde, drying the impregnated fabric at.70 C. and,after-hardening .the driedfabric at 9. A processhfor rendering textilefabr ics.flameproof,

which Comprisesimpregnating-the fabric in an. aqueous liquid whichcontainstl) rabout 24-3 8% byweig htpfa reaction mixture obtained \byheating an excesslo f urea together withmeta-P 1Osphoric;acid at a hightemperature,

amine, the ratio of, component (.1),,to1comppnent (2) ranging from. 3 :1to 6: 1, and dryingthe impregnated fabric at a temperature ranging from70 C.; to .1 60- C.

, 10,. A procession rendering-,textile fabrics flameproof,

mixture obtained by heating 8 parts of urea with 3 parts ofmeta-phosphoric acid at 160 C. for about 15 minutes, and (2) 9% byweight of a condensation product of unlimited water-solubility of onemol of melamine with about 2-3 mols of formaldehyde, drying theimpregnated fabric at 70 C. and after-hardening the dried fabric at C.

11. A textile fabric which has a flameproof dressing obtained byimpregnating the fabric in an aqueous liquid which contains (1) about12-48% by weight of a reaction mixture obtained by heating an excess ofurea together with an oxygen-containing compound of phosphorus selectedfrom the group consisting of ortho-phosphoric acid, pyrophosphoric acid,meta-phosphoric acid and ammonium salts of these acids, at a hightemperature, and (2) about 2-18% by weight of a methylol-melamine, theratio of component 1) to component (2) ranging from 3:1 to 6:1, anddrying the impregnated fabric at a raised temperature, the amounts ofphosphorus and nitrogen fixed in and on the fibers being about 13-40% ofthe former and about 2.8-6.0% of the latter.

12. A textile fabric which has a flameproof dressing obtained byimpregnating the fabric in an aqueous liquid which contains (1) about24-38% by weight of a reaction mixture obtained by heating an excess ofurea together with an oxygen-containing compound of phosphorus selectedfrom the group consisting of ortho-phosphoric acid, pyrophosphoric acid,meta-phosphoric acid and ammonium salts of these acids, at a hightemperature, and (2) about 4-12% by weight of a methylol-melamine, theratio of component (1) to component (2) ranging from 3:1 to 6:1, anddrying the impregnated fabric at a raised temperature, the amounts ofphosphorus and nitrogen fixed in and on the fibers being about 134.0%ofthe former and about 2.8-6.0% of the latter.

13. A textile fabric which has a flameproof dressing obtained byimpregnating the fabric in an aqueous liquid which contains (1) about24-38% by weight of a reaction mixture obtained by heating an excess ofurea together with pyrophosphoric acid at a high temperature, and (2)about 4-12% by weight of a methylol-melamine the ratio of component (1)to component (2) ranging from 3:1 to 6:1, and drying the impregnatedfabric at a temperature ranging from 70 C. to 160 C., the amounts ofphosphorus and nitrogen fixed in and on the fibers being about 1.34.0%of the former and about 2.8-6.0% of the latter.

14. A textile fabric which has a flameproof dressing obtained byimpregnating the fabric in an aqueous liquid which contains (1) about27-37% by weight of a reaction mixture obtained by heating -8 parts ofurea with 3 parts of pyrophosphoric acid at 160 C. for 30 minutes, and(2) 9% by weight of a condensation product of unlimited water-solubilityof one mol of melamine with about 2-3 mols of formaldehyde, drying theimpregnated fabric at 70 C. and after-hardening the dried fabric atISO-160 C., the amounts of phosphorus and nitrogen fixed in and on thefibers being 2.52.9% of the former and 4.8-5.3% of the latter.

15. A textile fabric which has a flameproof dressing obtained byimpregnating the fabric in an aqueous liquid which contains (1) about24-38% by weight of a reaction mixture obtained by heating an excess ofurea together with diammonium ortho-phosphate at a high temperature,and, (2) about 4-12% by weight of a methylol melamine, the ratio ofcomponent (1) to component (2) ranging from 3:1 to 6:1, and drying theimpregnated fabric at a temperature ranging from 70 C. to 160 C., theamounts of phosphorus and nitrogen fixed in and on the fibers beingabout 1.3-4.0% of the former and about 2.8-6.0% of the latter.

16. A textile fabric which has a flameproof dressing obtained byimpregnating the fabric in an aqueous liquid which contains (1) 38% byweight of a reaction mixture obtained by heating 6 parts of urea with 3parts of 'diammonium ortho-phosphate at 160 C. for 30 minutes, and (2)9% by weight of a condensation product unlimited water-solubility of onemol of melamine with about 2-3 mols of formaldehyde, drying theimpregnated fabric at C. and after-hardening the dried fabric at C., theamounts of phosphorus and nitrogen fixed in and on the fibers beingabout 1.7-1.8% of the former and about 4.142% of the latter.

17. A textile fabric which has a flameproof dressing obtained byimpregnating the fabric in an aqueous liquid which contains (1) about24-38% by Weight of a reaction mixture obtained by heating an excess ofurea together with ortho-phosphoric acid at a high temperature, and (2)about 4-12% by weight of a methylol-melamine, the ratio of component (1)to component (2) ranging from 3:1 to 6:1, and drying the impregnatedfabric at a temperature ranging from 70 C. to 160 C., the amounts ofphosphorus and nitrogen fixed in and on the fibers being about 1.3-4.0%of the former and about 2.8-6.0% of the latter.

18.A textile fabric which has a fiameproof dressing obtained byimpregnating the fabric in an aqueous liquid which contains (1) 32% byweight of a reaction mixture obtained by heating 6 parts of urea with 3parts of orthophosphoric acid at 160 C. for 10 minutes, and, (2) 9% byweight of a condensation product of unlimited watersolubility of one molof melamine with about 2-3 mols of formaldehyde, drying the impregnatedfabric at 70 C. and after-hardening the dried fabric at C., the amountsof phosphorus and nitrogen fixed in and on the fibers being about 2.1%of the former and about 5.0% of the latter.

Reference Cited in the file of this patent UNITED STATES PATENTS2,089,697 Groebe Aug. 10, 1937 2,305,035 Rosser Dec. 15, 1942 2,452,054Jones et al. Oct. 26, 1948 2,464,342 Pollak Mar. 15, 1949 2,488,034Pingree Nov. 15, 1949 2,537,840 McLean Jan. 9, 1951 FOREIGN PATENTS638,434 Great Britain June 7, 1950

1. A PROCESS FOR RENDERING TEXTILE FABRICS FLAMEPROOF, WHICH COMPRISESIMPREGNATING THE FABRIC IN AN AQUEOUS LIQUID WHICH CONTAINS (U) ABOUT12-48% BY WEIGHT OF A REACTION MIXTURE OBTAINED BY HEATING AN EXCESS OFUREA TOGETHER WITH AN OXYGEN-CONTAINING COMPOUND OF PHOSPHORUS SELECTEDFROM THE GROUP CONSISTING OF ORTHOPHOSPHORIC ACID, PYROPHOSPHORIC ACID,META-PHOSPHORIC ACID AND AMMONIUM SALTS OF THESE ACIDS, AT A HIGHTEMPERATURE, AND (2) ABOUT 2-18% BY WEIGHT OF A METHYLOLMELAMINE, THERATIO OF COMPONENT (1) TO COMPONENT (2) RANGING FROM 3:1 TO 6:1, ANDDRYING THE IMPREGNATED FABRIC AT A RAISED TEMPERATURE.